If you have less consumers than partitions, does that simply mean you will not consume all the messages on a given topic?
In a cloud environment, how are you suppose to keep track how many consumers are running and how many are pointing to a given topic#partition?
What if you have multiple consumers on a given topic#partition? I guess the consumer has to somehow keep track of what messages it has already processed in case of duplicates?
In fact, each consumer belongs to a consumer group. When Kafka cluster sends data to a consumer group, all records of a partition will be sent to a single consumer in the group.
If there're more paritions than consumers in a group, some consumers will consume data from more than one partition. If there're more consumers in a group than paritions, some consumers will get no data. If you add new consumer instances to the group, they will take over some partitons from old members. If you remove a consumer from the group (or the consumer dies), its partition will be reassigned to other member.
Now let's take a look at your questions:
If you have less consumers than partitions, does that simply mean you will not consume all the messages on a given topic?
NO. Some consumers in the same consumer group will consume data from more than one partition.
In a cloud environment, how are you suppose to keep track how many consumers are running and how many are pointing to a given topic#partition?
Kafka will take care of it. If new consumers join the group, or old consumers dies, Kafka will do reblance.
What if you have multiple consumers on a given topic#partition?
You CANNOT have multiple consumers (in a consumer group) to consume data from a single parition. However, if there're more than one consumer group, the same partition can be consumed by one (and only one) consumer in each consumer group.
1) No that means you will one consumer handling more than one consumer. 2) Kafka never assigns same partition to more than one consumer because that will violate order guarantee within a partition. 3) You could implement ConsumerRebalanceListener, in your client code that gets called whenever partitions are assigned or revoked from consumer.
You might want to take a look at this article specically "Assigning partitions to consumers" part. In that i have a sample where you create topic with 3 partitions and then a consumer with ConsumerRebalanceListener telling you which consumer is handling which partition. Now you could play around with it by starting 1 or more consumers and see what happens. The sample code is in github
http://www.javaworld.com/article/3066873/big-data/big-data-messaging-with-kafka-part-2.html
